Because of its long life and its small signal distortion, a telephone transmitter containing an amplifier is suitable for data communication in which a telephone transmitter is combined with an acoustic coupler. Therefore, transmitters with amplifiers have been finding an increasing use in displacing of the conventional carbon transmitter, as a result of the progress in data communication.
In the conventional telephone transmitter amplifiers, it is the practice to use Darlington circuits for the reduction of fluctuations in the D.C. resistance of the telephone transmitter, caused by the fluctuation in the D.C. current amplification factor. The transistors used in the Darlington connection usually are two silicon NPN transistors or the complementary Darlington connection of PNP and NPN transistors at the out put stage of the amplifier, as taught in ("Electronic Telephone Sets" By A, Boeryd, ERICSSON REVIEW, No. 1, pp. 2-12, 1974).
While the Darlington connection is suited for a low current operation and effective for the suppression of D. C. resistance fluctuation the minimum ON voltage Von is high. When used in a parallel connection with the low-resistance telephone set, however, the voltage across the telephone transmitter is lowered, thereby unavoidably lowering the maximum distortionless output. More particularly, with the voltage across a telephone transmitter represented by E.sub.T, the maximum distortionless output in peak-to-peak value is represented by 2 (E.sub.T -Von). Therefore, if the voltage E.sub.T is reduced to about 1 V and the voltage Von is assumed to be 0.7 V, the maximum distortionless output is lowered to 0.6 V in peak-to-peak value.
In addition, the reduction in the supply voltage across the telephone transmitter results in the decrease in the arriving signal level at a telephone exchange. To avoid this, it has been proposed to connect a field effect transistor for adjusting the output level on the input side of the preamplifier (Reference is made to "An Integrated Speech Circuit" (in Japanese) by M. Terai, et al, ELECTRICAL COMMUNICATION LABORATORIES TECHNICAL JOURNAL, Vol. 22, No. 2, pp. 261-284, 1973). However, this approach has a disadvantage in that the differences in the threshold voltage from one field effect transistor to another requires a fine adjustment for each transistor, thereby complicating the process of manufacturing such a telephone transmitter amplifier in an integrated circuit form.
It is therefore an object of the present invention to provide a telephone transmitter amplifier capable of providing a high maximum distortionless output voltage even with a lowered supply voltage from the telephone line and of maintaining a constant gain even at a low current level.
Another object of the present invention is to provide a telephone transmitter amplifier having an automatically controlled output level.
Still another object of the present invention is to provide a telephone transmitter amplifier of minimized power consumption.
Further object of the present invention is to provide a telephone transmitter amplifier suited for manufacture in an integrated circuit form.